Mathew Martoma had been on his way to being another American success story. Today, he is the eighth person who once worked for the hedge fund titan Steven A. Cohen to be convicted of insider trading. A federal jury in Manhattan on Thursday found Mr. Martoma guilty of seeking out confidential information related to a clinical trial for an experimental drug for Alzheimer’s disease — information that enabled Mr. Cohen’s SAC Capital Advisors to earn $275 million.